240 SILURIAN INLTER IN THE EASTERN MENDIPS. [Ma)' 1907. 



conformity was prophetic of the Malvernian movements. The 

 association of volcanic activity with the anticipatory folding was 

 of great interest. In the Cardiff area which lay to the north of 

 the Armorican margin, the Silurian was complete as regarded its 

 upper members, and passed insensibly into the Old lied Sandstone, 

 so that in this locality sedimentation was continuous. A sub- 

 sidence might possibly have occurred there in complement of the 

 rise on the south. It must, however, be added that the deter- 

 mination of the horizon of the Men dip Silurian rested on extremely 

 uncertain data. Most of the fossils were of wide range, and the 

 general facies recalled that of the Ludlow, near lihymney. Obser- 

 vations on volcanoes in a persistently-active state as Vulcano was 

 for a considerable period, afforded reason to believe that the rounding 

 of fragments in an agglomerate might be produced as a consequence 

 of repeated explosions. 



Prof. W. W. Watts congratulated the Author on having at last 

 definitely found that in Britain there were volcanic rocks of Silurian 

 age. He suggested that the difficulty as to the exact age of the 

 rocks, whether Llandovery, Wenlock, or Lower Ludlow, might be 

 solved if it was remembered that the Llandovery formation was 

 laid down during subsidence of an old land-surface. Hence the 

 deposits crept up the flank of the old land, and deposits of precisely- 

 similar lithological character might be of different ages, and yet 

 retain a similar facies of fossils, related to the Llandovery Beds 

 because of the similarity of deposit, but related to the Wenlock or 

 Lower Ludlow because of identity in time of deposition. 



Mr. J. Y. Elsden said that he noticed that, whereas the 

 contemporaneous lavas appeared to be pyroxene- andesites, the 

 spherical fragments in the coarse, ashy conglomerate, judging from 

 the microscope-section exhibited, appeared to be of a different, 

 although allied, type. He would like to ask the Author whether 

 any of the neighbouring lava- flows resembled the latter type. 



The Author thanked the Fellows for the favourable reception 

 that they had accorded to his paper. He was glad to know that 

 the Chairman and Prof. Sollas saw nothing inconsistent with the 

 ' neck theory ' for the coarse ashy conglomerate in the well-rounded 

 character of the majority of the blocks. Prof. Sollas's comparison 

 of the Mendip and Tortworth areas with that of Cardiff was very 

 interesting, and it was unfortunate that the Mendip Silurian fossils 

 did not afford more conclusive evidence as to the exact age of the 

 deposit. The Author welcomed Prof. W r atts'"s suggestion that the 

 peculiar character of the fossils might be due to the deposit having 

 been laid down in shallow water. In reply to Mr. Winwood, he 

 said that the volcanic episode in the Tortworth area belonged, in all 

 probability, entirely to Llandovery time. In reply to Mr. Elsden's 

 enquiry concerning the ashy fragments in the tuffs, the Author 

 stated that they showed a considerable amount of variability, and 

 were sometimes of types not precisely to be paralleled among the 

 neighbouring lava-flows. The majority were of the same nature as 

 the associated andesitic lavas, but some were more basic, and others 

 were suggestive of trachyte. The dark coloration of many was due 

 to the abundance of ferric oxide. 



